A typical vane-type camshaft phaser for changing the phase relationship between a crankshaft and a camshaft of an internal combustion engine generally comprises a plurality of outwardly-extending vanes on a rotor interspersed with a plurality of inwardly-extending lobes on a stator, forming alternating advance and retard chambers between the vanes and lobes. Engine oil is selectively supplied to one of the advance and retard chambers and vacated from the other of the advance chambers and retard chambers by a phasing oil control valve in order to rotate the rotor within the stator and thereby change the phase relationship between the camshaft and the crankshaft. One such camshaft phaser is described in U.S. Pat. No. 8,534,246 to Lichti et al., the disclosure of which is incorporated herein by reference in its entirety and hereinafter referred to as Lichti et al. As is typical for phasing oil control valves, the phasing oil control valve of Lichti et al. operates on the principle of direction control, i.e. the position of the oil control valve determines the direction of rotation of the rotor relative to the stator. More specifically, when a desired phase relationship between the camshaft and the crankshaft is determined, the desired phase relationship is compared to the actual phase relationship as determined from the outputs of a camshaft position sensor and a crankshaft position sensor. If the actual phase relationship, does not match the desired phase relationship, the oil control valve is actuated to either 1) an advance position to supply oil to the retard chambers and vent oil from the advance chambers or 2) a retard position to supply oil to the advance chambers and vent oil from the retard chambers until the actual phase relationship matches the desired phase relationship. When the actual phase relationship matches the desired phase relationship, the oil control valve is positioned to hydraulically lock the rotor relative to the stator. However, leakage from the advance chambers and the retard chambers or leakage from the oil control valve may cause the phase relationship to drift over time. When the drift in phase relationship is detected by comparing the actual phase relationship to the desired phase relationship, the oil control valve must again be actuated to either the advance position or the retard position in order to correct for the drift, then the oil control valve is again positioned to hydraulically lock the rotor relative to the stator after the correction has been made. Consequently, the position of the rotor relative to the stator is not self-correcting and relies upon actuation of the phasing oil control valve to correct for the drift.
U.S. Pat. No. 5,507,254 to Melchior, hereinafter referred to as Melchior, teaches a camshaft phaser with a phasing oil control valve which allows for self-correction of the rotor relative to the stator as may be necessary due to leakage from the advance chamber or from the retard chamber. Melchior also teaches that the valve spool defines a first recess and a second recess separated by a rib such that one of the recesses acts to supply oil to the advance chamber when a retard in timing of the camshaft is desired while the other recess acts to supply oil to the retard chamber when an advance in the timing of the camshaft is desired. The recess that does not act to supply oil when a change in phase is desired does not act as a flow path. However, improvements are always sought in any art.
What is needed is a camshaft phaser which minimizes or eliminates one or more the shortcomings as set forth above.